In the operation of a turbo machine attempts have been made to remain within the so-called stable area and, in particular, not to cross the boundary line with the unstable area. This boundary line is also known as the stalling limit and in practice is frequently referred to as the surge line. In that case a strong areodynamic pulsation occurs, which may cause mechanical damage to the turbo machine. A turbo machine is always so designed as to function amply within the stable area. However, in a turbo machine it is often impossible to avoid crossing the boundary line between the stable and unstable area when the quantity of fluid flowing through the machine is reduced.